1. Field of the Invention
This invention relates to an apparatus for depositing a thin film on a substrate, and more particularly to an apparatus for depositing a thin film onto a substrate disposed in a vacuum vessel by ionizing a cluster beam emitted from an evaporation source and causing the ionized cluster beam to impinge on the substrate.
2. Description of the Prior Art
FIG. 1 shows a sectional view of a conventional thin film deposition system, as for instance disclosed in Japanese Patent Publication No. 54-9592 (1979). In the figure, there are shown a vacuum vessel maintained at a predetermined degree of vacuum, and an evacuation passage 2 for evacuating the vessel 1, the passage being connected to an evacuation device (not shown). A crucible 3 has a lid 3a provided with one or more nozzles 3b having an inside diameter of 1 to 2 mm. The crucible 3 is charged with an evaporation material 4, for instance, aluminum. A crucible support rod 5 is attached to a crucible support base 7 through an insulating support member 6. Means 8 for heating the crucible 8 is provided which is, for instance, a heating filament. A thermal shield plate 9 is provided for intercepting the radiant heat from the heating filament 8. The crucible 3, the heating filament 8 and the thermal shield plate 9 constitute an evaporation source 12 for heating and vaporizing the evaportion material 4 to form a cluster beam 17 consisting of clusters 26 of the evaporation material. A base plate 10 is attached to the vacuum vessel 1 by a support 11.
Denoted by 13 is a thermionic emission portion for emitting thermoelectrons 14 for ionization. The thermionic emission portion 13 comprises, as shown in FIG. 2, an ionizing filament 15, and an electron extraction electrode 16 for accelerating the thermoelectrons 14, emitted from the filament 15, so as to irradiate the cluster beam 27 with the thermoelectrons 14. Denoted by 17 is a shield plate for intercepting the heat radiated from the ionizing filament 15. The thermionic emission portion 13 and the shield plate 17 constitute ionizing means 18 for ionizing the cluster beam 27 emitted from the evaporation source 12. The ionizing filament 15 and the electron extraction electrode 16 are usually disposed on a cylindrical, conical or axisymmetric polygonal surface surrounding the cluster beam 27. The filament 15 and the extraction electrode 16 are located at positions corresponding to an ejecting angle of the cluster beam, as measured from the center of the beam, of 10 to 20 degrees. An acceleration electrode 19 is provided so that a voltage can be applied between the acceleration electrode 19 and the electron extraction electrode 16. An insulating support member 20 supports the extraction electrode 16 and the acceleration electrode 19 in an insulating manner. The insulating support member 20 is attached to the base plate 10 by a pillar 21 and an insulator 22. A substrate 23 is disposed in the vacuum vessel 1 by use of a substrate holder 24 and an insulating member 25. Numeral 28 denotes the ionized clusters, namely, the clusters ionized by the ionizing means 18.
The conventional thin film deposition system operates as follows.
In the case of evaporating and depositing a thin film of aluminum onto the substrate 23, aluminum 4 as the evaporation material is first placed in the crucible 3, and the vacuum vessel 1 is evacuated by the evacuating device to a vacuum degree of about 10.sup.-4 Pa (about 10.sup.-6 Torr). Next, the heating filament 8 is supplied with a current to generate heat, and aluminum 4 in the crucible 3 is heated to vaporize, either by the radiant heat from the filament 8 or by collision of the thermoelectrons emitted from the filament 8. When the crucible 3 is heated up to such a temperature that the vapor pressure of aluminum 4 is about 10 to 10.sup.3 Pa (0.1 to several tens of Torr), aluminum vapor is ejected through the nozzle 3b. At this moment, the pressure difference between the inside of the crucible 3 and the interior of the vacuum vessel 1 causes adiabatic expansion of the aluminum vapor, resulting in the formation of the cluster beam 27 consisting of the so-called clusters, that is, aggregates of a multiplicity of atoms loosely bound together.
The cluster beam 27 is irradiated with the thermoelectrons 14 extracted from the ionizing filament 15 toward the center axis of the cluster beam 27 by the electron extraction electrode 16. By the irradiation, some clusters in the cluster beam are converted into the ionized clusters 28 through the ionization of one atom in the cluster. Thus the ionized clusters 28 are accelerated appropriately by an electric field developed between the acceleration electrode 19 and the electron extraction electrode 16, to collide against the substrate 23 with the kinetic energy acquired from the field in addition to that acquired when the unionized neutral clusters 26 are ejected from the crucible 3. As a result, a thin film of aluminum is evaporated onto the substrate 23.
In the conventional thin film deposition system constructed as above, the ionizing filament 15 and the electron extraction electrode 16 constituting the thermionic emission portion are disposed on a cylindrical surface which surrounds the cluster beam 27. Therefore, an upper portion of the thermionic emission potion 13 is exposed constantly to the cluster beam 27, and, when the intensity of the cluster beam is increased in order to obtain a higher evaporation rate, the cluster beam would be condensed on the electron extraction electrode 16 or the like to cause a reaction of the clusters with the electrode material or deposition of the evaporation material 4 on the electrode 16 or the like, leading to problems such as rapid deformation of the electrode 16. To obivate the problems, it has been necessary to replace the ionizing means 18 frequently. In addition, there has been the possibility of a shortcircuit accident being caused by a large deformation of the electron extraction electrode 16 or the like.
Besides, ionization of a cluster beam with high intensity has involved difficulties in the control of ionizing conditions, because of generation of a plasma between the ionizing filament 15 and the electron extraction electrode 16 due to a high vapor density at the thermionic emission portion. Particularly in the case of using an element having a small work function, such as barium, as the evaporation material 4, deposition of the element on the ionizing filament 15 being at a high temperature would lower the work function on the filament surface, causing a rapid increase in the thermionic emission quantity and, hence, troubles such as breakage of a power supply for ionization.
On the other hand, an increase in the diameter of the thermionic emission portion 13 in order to prevent the irradiation of the thermionic emission portion 13 with the cluster beam involves an increase in the outside dimensions of the ionizing means 18, making it impossible to construct a compact system advantageous on the basis of evacuation time and cost.
As a method of controlling the irradiation of the thermionic emission portion 13 with the cluster beam, use of a thermionic emission portion formed in a divergent conical shape is easy to think out. In fact, there has been known an ionizing means, as shown in FIG. 3, described in the literature: W. Knauer and R. L. Poeschei, Ionized cluster beam deposition, J. Vac. Sci. Technol., B6(1), 1988, pp. 456-460. This system, originally used for analytical studies of rare gas cluster beams, was converted to an ionizing means for film deposition, in which a cluster beam 27a passing through a hole 4 in a shield plate 40 is caused to pass through the inside of divergent electron extraction electrodes 16 and an ionizing filament 15. The means, however, has problems as to machining technique, for instance, difficulties in shaping a plurality of conical electron extraction electrodes accurately and in assembling the electrodes with good reproducibility.